In the space between the stars, in the dust from which they are born, scientists have found molecules that have never before been seen in space.
The discovery of tell-tale emissions in a dense star-forming cloud marks the first time specific species of a class of molecules known as polycyclic aromatic hydrocarbons have been identified in space – and it could help finally unravel the mystery of how and where these molecules form.
“We had always thought polycyclic aromatic hydrocarbons were primarily formed in the atmospheres of dying stars,” said MIT astrochemist Brett McGuire.
“In this study, we found them in cold, dark clouds where stars haven’t even started forming yet.”
Polycyclic aromatic hydrocarbons, or PAHs, aren’t exactly rare. Here on Earth, they show up a lot, without even waiting for an invitation. They can be found in crude oil – made of compressed and superheated dead organisms such as zooplankton and algae; and coal – made of compressed and superheated dead plants.
The origins of PAHs can be non-biological, too – in fact, as far as we know, most of the PAHs in the Universe are from abiotic origin. Previous analysis suggests around 15 percent of all carbon between the stars in galaxy’s like our own is bound up in PAHs – and most of that is floating in space between the stars, in the interstellar medium.
However, all we had was a bulk signature telling us that the PAHs are out there, not which individual molecules can be found in the interstellar gas.
“Now, for the first time, we have a direct window into their chemistry that will let us study in detail how this massive reservoir of carbon reacts and evolves through the process of forming stars and planets,” McGuire said.
The focus of the team’s research was the Taurus Molecular Cloud (TMC-1), which is what it sounds like – a cold, dense molecular cloud in the Taurus region of the sky, about 430 light-years away.
The team has been scouring this cloud for PAHs for some time as part of a project named GOTHAM – Green Bank Telescope (GBT) Observations of TMC-1: Hunting Aromatic Molecules.
Molecular clouds are where baby stars are born, when a dense knot in the gas collapses under its own gravity while spinning, spooling more material onto itself from the cloud around it.
What the team found inside TMC-1, however, was not what the models predicted. Obviously some PAHs were expected – but the abundances were orders of magnitude higher than expected.
Today’s paper in Science details the detection of 1- and 2-cyanonaphthalene, but other papers in recent months by McGuire and his colleagues reveal the tremendous wealth of what they have uncovered using their data-stacking techniques: HC4NC, benzonitrile, propargyl cyanide, HC11N, cyanocyclopentadiene, 2-cyanocyclopentadiene, trans-cyanovinylacetylene, and vinylcyanoacetylene.
“We’ve stumbled onto a whole new set of molecules unlike anything we’ve previously been able to detect, and that is going to completely change our understanding of how these molecules interact with each other. It has downstream ramifications,” McGuire said.
“When these molecules get big enough that they’re the seeds of interstellar dust, these have the possibility then to affect the composition of asteroids, comets, and planets, the surfaces on which ices form, and perhaps in turn even the locations where planets form within star systems.”
The team doesn’t currently know precisely how so much 1- and 2-cyanonaphthalene ended up in TMC-1 – none of their models were unable to explain the abundance. The two possibilities are that it formed there, or that it was transported there; or maybe a combination of both.
Now that the detection is made, though, and we know that it’s there, scientists can get to work on figuring that part out. We know it’s there; we know it had to get there somehow; figuring out the rest is usually only a matter of time.
The discovery also shows that the chemistry of molecular clouds could be way more rich and complex than we thought, and gives us a new tool for understanding our Universe.
“The amazing thing about these observations, about this discovery, and about these molecules, is that no one had looked, or looked hard enough,” said astrochemist Michael McCarthy of the Harvard & Smithsonian Center for Astrophysics. “It makes you wonder what else is out there that we just haven’t looked for.”
The research has been published in Science.
